1. Field of the Invention
The present invention relates to a micro mirror unit and a method of making it. The micro mirror unit is an element incorporated e.g. in an optical switching device which switches optical paths between a plurality of optical fibers, or in an optical disc drive which records data onto an optical disc and/or reproduces data recorded on it.
2. Description of the Related Art
In recent years, optical communications technology is utilized widely in a variety of fields. In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used in general. In order to achieve high quality optical communications, the optical switching device must have high capacity, high speed and high reliability in switching action. In view of these, micro mirror units manufactured by micro-machining technology is attracting attention as a switching element to be incorporated in the optical switching device. The micro mirror units enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous to achieving the desired characteristics mentioned above.
Micro mirror units are disclosed e.g. in Japanese Patent Laid-Open No. 4-343318 and No. 11-52278. Further, optical switching devices which use micro mirror units manufactured by micro-machining technologies are disclosed in the article “MEMS Components for WDM Transmission Systems” (Optical Fiber Communication [OFC] 2002, pp.89–90 etc.
FIG. 21 outlines an ordinary optical switching device 500. The optical switching device 500 includes a pair of micro mirror arrays 501, 502, an input fiber array 503, an output fiber array 504, and a plurality of micro lenses 505, 506. The input fiber array 503 includes a predetermined number of input fibers 503a. The micro mirror array 501 is provided with the same plurality of micro mirror units 501a each corresponding to one of the input fibers 503a. Likewise, the output fiber array 504 includes a predetermined number of input fibers 504a. The micro mirror array 502 is provided with the same plurality of micro mirror units 502a each corresponding to one of the output fibers 504a. Each of the micro mirror units 501a, 502a has a mirror surface for reflection of light. The orientation of the mirror surface is controllable. Each of the micro lenses 505 faces an end of a corresponding input fiber 503a. Likewise, each of the micro lenses 506 faces an end of a corresponding output fiber 504a. 
In transmitting optical signals, lights L1 coming out of the output fibers 503a pass through the corresponding micro lenses 505 respectively, thereby becoming parallel to each other and proceeding to the micro mirror array 501. The lights L1 reflect on their corresponding micro mirror units 501a respectively, thereby deflected toward the micro mirror array 502. At this point, the mirror surfaces of the micro mirror units 501a are oriented, in advance, in predetermined directions so as to direct the lights L1 to enter their respective desired micro mirror units 502a. Then, the lights L1 are reflected on the micro mirror units 502a, and thereby deflected toward the output fiber array 504. At this point, the mirror surfaces of the micro mirror units 502a are oriented, in advance, in predetermined directions so as to direct the lights L1 into their respective desired output fibers 504a. 
As described, according to the optical switching device 500, the lights L1 coming out of the input fibers 503a reach the desired output fibers 504a due to the deflection by the micro mirror arrays 501, 502. In other words, a given input fiber 503a is connected with an output fiber 504a in a one-to-one relationship. With this arrangement, by appropriately changing deflection angles of the micro mirror units 501a, 502a, switching can be performed and the lights L1 can be deflected into different output fibers 504a. 
FIG. 22 outlines another ordinary optical switching device 600. The optical switching device 600 includes a micro mirror array 601, a fixed mirror 602, an input-output fiber array 603, and a plurality of micro lenses 604. The input-output fiber array 603 includes a predetermined number of input fibers 603a and a predetermined number of output fibers 603b. The micro mirror array 601 includes the same plurality of micro mirror units 601a each corresponding to one of the fibers 603a, 603b. Each of the micro mirror units 601a has a mirror surface for reflection of light and orientation of the mirror surfaces is controllable. Each of the micro lenses 604 faces an end of a corresponding one of the fibers 603a, 603b. 
In transmitting optical signals, light L2 coming out of the input fiber 603a passes through the corresponding micro lens 604 and is directed toward the micro mirror array 601. The light L2 is then reflected by a corresponding first micro mirror unit 601a, and thereby deflected toward the fixed mirror 602, reflected by the fixed mirror 602, and then enters a corresponding second micro mirror unit 601a. At this point, the mirror surface of the first micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a predetermined one of the micro mirror units 601a. Then, the light L2 is reflected on the second micro mirror unit 601a, and thereby deflected toward the input-output fiber array 603. At this point, the mirror surface of the second micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a predetermined one of the output fibers 603b. 
As described, according to the optical switching device 600, the light L2 coming out of the input fiber 603a reaches the desired output fiber 603b due to the deflection by the micro mirror array 601 and the fixed mirror 602. In other words, a given input fiber 603a is connected with an output fiber 603b in a one-to-one relationship. With this arrangement, by appropriately changing deflection angles of the first and the second micro mirror units 601a, switching can be performed and the light L2 can be deflected into different output fibers 603b. 
FIG. 23 is a perspective view, partly unillustrated, of a portion of a conventional micro mirror unit 700 for incorporation in such devices as the optical switching devices 500, 600. The micro mirror unit 700 includes a mirror-formed portion 710 having an upper surface provided with a mirror surface (not illustrated), an inner frame 720 and an outer frame 730 (partly unillustrated), each formed with come-like electrodes integrally therewith. Specifically, the mirror-formed portion 710 has ends facing away from each other, and a pair of comb-like electrodes 710a, 710b are formed respectively on these ends. In the inner frame 720 a pair of comb-like electrodes 720a, 720b extend inwardly, corresponding to the comb-like electrodes 710a, 710b. Also, a pair of comb-like electrodes 720c, 720d extend outwardly. In the outer frame 730 a pair of comb-like electrodes 730a, 730b extend inwardly, corresponding to the comb-like electrodes 720c, 720d. The mirror-formed portion 710 and the inner frame 720 are connected with each other by a pair of torsion bars 740. The inner frame 720 and the outer frame 730 are connected with each other by a pair of torsion bars 750. The pair of torsion bars 740 provides a pivotal axis for the mirror-formed portion 710 to pivot with respect to the inner frame 720. The pair of torsion bars 750 provides a pivotal axis for the inner frame 720, as well as for the associating mirror-formed portion 710, to pivot with respect to the outer frame 730.
With the above arrangement, in the micro mirror unit 700, a pair of comb-like electrodes, such as the comb-like electrode 710a and the comb-like electrode 720a, are opposed closely to each other for generation of static electric force, and take positions as shown in FIG. 24A, i.e. one of the electrode assuming a lower position and the other assuming an upper position, when there is no voltage applied. When an electric voltage is applied, as shown in FIG. 24B, the comb-like electrode 710a is drawn toward the comb-like electrode 720a, thereby pivoting the mirror-formed portion 710. More specifically, in FIG. 23, when the comb-like electrode 710a is given a positive charge whereas the comb-like electrode 720a is given a negative charge, the mirror-formed portion 710 is pivoted in a direction M1 while twisting the pair of torsion bars 740. On the other hand, when the comb-like electrode 720c is given a positive charge whereas the comb-like electrode 730a is given a negative charge, the inner frame 720 is pivoted in a direction M2 while twisting the pair of torsion bars 750.
As a conventional method, the micro mirror unit 700 can be made from an SOI (Silicon on Insulator) wafer which sandwiches an insulating layer between silicon layers. Specifically, first, as shown in FIG. 25A, a wafer 800 is prepared which has a layered structure including a first silicon layer 801, a second silicon layer 802, and an insulating layer 803 sandwiched between these silicon layers. Next, as shown in FIG. 25B, an anisotropic etching is performed to the first silicon layer 801 via a predetermined mask, to form the mirror-formed portion 710, torsion bars 140, the comb-like electrode 710a and other members to be formed on the first silicon layer 801. Next, as shown in FIG. 25C, an anisotropic etching is performed to the second silicon layer 802 via a predetermined mask, to form the comb-like electrode 720a and other members to be formed on the second silicon layer 802. Note that for the sake of simplification of the drawings, each of the FIG. 25A through FIG. 25C gives only one sectional view, and each view includes a plurality of sections taken at different locations in the wafer 800.
However, according to the conventional method of manufacture as described above, the thickness of the wafer 800 is directly reflected on the thickness of the micro mirror unit 700. Specifically, the thickness of the micro mirror unit 700 is identical with the thickness of the wafer 800 which is used for the formation of the micro mirror unit. For this reason, according to the conventional method, the material wafer 800 must have the same thickness as the thickness of the micro mirror unit 700 to be manufactured. This means that if the micro mirror unit 700 is to be thin, the wafer 800 of the same thinness must be used. For example, take a case of manufacturing a micro mirror unit 700 having a mirror surface having a size of about 100 through 1000 μm. In view of a mass of the entire moving part including the mirror-formed portion 710 and the inner frame 720, the amount of movement of the moving part, the size of the comb-like electrodes necessary for achieving the amount of movement, etc considered comprehensively, a desirable thickness of the moving part or the micro mirror unit 700 is determined. In this particular case the desirable thickness is 100 through 200 μm. As a result, in order to manufacture the micro mirror unit 700 having such a thickness, a wafer 800 having the thickness of 100 through 200 μm is used.
According to the conventional method, in order to manufacture a thin micro mirror unit 700, a correspondingly thin wafer 800 must be used. This means that the greater diameter the wafer 800 has, the more difficult to handle the wafer. For instance, take a case in which a micro mirror unit 700 is to be manufactured from an SOI wafer 800 having a thickness of 200 μm and a diameter of 6 inches. Often, the wafer 800 is broken in a midway of the manufacturing process. After formation of the predetermined structural members on the first silicon layer 801 as shown in FIG. 25B, strength of the wafer 800 is decreased, making especially difficult to handle the wafer during the machining on the second silicon layer 802. Thinness of the wafer 800 limits, as has been described, the size of the flat surface of the wafer due to handling difficulties. Further, the limitation on the size of the flat surface of the wafer places a limitation on the manufacture of micro mirror array chips. Specifically, when the micro mirror array chips are manufactured by forming a plurality of micro mirror units in an array pattern on a single substrate, the size of the array is limited.
FIG. 26 shows a micro mirror unit 700 mounted on a wiring substrate. In the figure, the micro mirror unit 700 shows a section taken on lines XXVI—XXVI in FIG. 23. According to the conventional micro mirror unit 700 in FIG. 23, the moving part including the mirror-formed portion 710 and the inner frame 720 has the same thickness as the outer frame 730. For this reason, when the micro mirror unit 700 is mounted onto the wiring substrate 810, in order to allow the moving part to move properly, a spacer 811 must be provided as shown in FIG. 26 between the wiring substrate 810 and the outer frame 730. By providing the spacer 811 having a sufficient thickness between the micro mirror unit 700 and the wiring substrate 810, it becomes possible to avoid a situation that the moving part makes contact to the wiring substrate 810 to become unable to move. In view of a mounting process of the micro mirror unit 700 onto the wiring substrate 810, it is not efficient to provide the spacer 811 separately.